I recently removed my unit test directory from the sonar.sources property and added it to sonar.tests. This resolved a number of issues that were being attributed to those files from the main dashboard view. However they still show up under the issues tab even when filtering on open (as can be seen in the screenshot). I restarted the server, re-ran the scan, etc. but no help.

the issues have status closed now. You will see this issues until they are deleted.
The default setting (sonar.dbcleaner.daysBeforeDeletingClosedIssues ) for deletion of closed issues is 30 days. You may change this settings here https://yoursonarhost/admin/settings / Delete closed issues after
Normally with Open issues filter they should not be seen. Did you also try with a more recent version of Sonarqube ?
Maybe this will help : stop Sonarqube, delete the %SONARQUBE_HOME%/data/es folder and start Sonarqube again. This will cause an elasticsearch reindexing.